At 06:07 PM 3/23/00 PST, you wrote:
>Can you generate a variable voltage with a capacitive multiplier and
>PWM?  My intuition says that PWM into a capacitor gives you sort of
>variable output current at a fixed voltage (at least for non-constant
>loads (like the Vpp input of an EPROM based PIC?))  Inductors give you
>variable voltage with PWM ?
>
>BillW
>

BillW,

Your intuition about PWM into a capacitor exactly matches my
conclusions after fiddling with the charge pump on the bench
the other day. Provided the cap isn't too large a value, it will
fully charge to the voltage placed across it, ie, 5v - 0.7v (for
diode) in the time allotted (1/78 Khz). However, the "effective"
current you can get out of the pump will be proportional to
(PWM duty cycle * max current from the pin). So PWMing this way
actually modulates the pump output current, rather than the pump
output voltage. Not very useful. [I didn't test this directly,
but infer this from my fiddling].

In the case where I produced a variable Vpp, I charged the 1st cap
in the pump chain directly off the PWM thru a 100 ohm resistor.
This way, the PWM duty cycle sets the average voltage the 1st cap
charges to, and ultimately the Vout of the pump. I presume
[but didn't measure it] that the effective pump current will be
about the same as before.

It would probably take 3 pins to do a decent job at variable Vpp.
One PWM to control V, and 2 running at 50% duty and opposite phase
to provide boost. But still you should be able to can get both +V
and -V out of this with the same #of pins [haven't tried it yet].

Separately adjustable +V and -V would require 4 pins, ie, 2 PWM
plus 2 opposite phase boosters.

If you wanted to use closed loop FB via A/D to keep the Vouts
constant with changing load, you'd need to go this last route
with 2 PWMs, so 6 pins total.

Lastly, your intuition about inductor response vs cap response,
viz-a-viz PWM, sounds accurate.

regards,
- Dan Michaels
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